[PATCH v2] clocksource: timer-riscv: Clear timer interrupt on timer initialization

[Samuel Holland]

On 2024-01-29 1:26 AM, Ley Foon Tan wrote:
> In the RISC-V specification, the stimecmp register doesn't have a default
> value. To prevent the timer interrupt from being triggered during timer
> initialization, clear the timer interrupt by writing stimecmp with a
> maximum value.
> 
> Fixes: 9f7a8ff6391f ("RISC-V: Prefer sstc extension if available")
> Cc:  <stable at vger.kernel.org>
> Signed-off-by: Ley Foon Tan <leyfoon.tan at starfivetech.com>
> 
> ---
> v2:
> Resolved comments from Anup.
> - Moved riscv_clock_event_stop() to riscv_timer_starting_cpu().
> - Added Fixes tag
> ---
>  drivers/clocksource/timer-riscv.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index e66dcbd66566..672669eb7281 100644
> --- a/drivers/clocksource/timer-riscv.c
> +++ b/drivers/clocksource/timer-riscv.c
> @@ -116,6 +116,9 @@ static int riscv_timer_starting_cpu(unsigned int cpu)
>  		ce->rating = 450;
>  	clockevents_config_and_register(ce, riscv_timebase, 100, 0x7fffffff);
>  
> +	/* Clear timer interrupt */
> +	riscv_clock_event_stop();

This change breaks boot on Unmatched. The bug is that the above call to
clockevents_config_and_register() sets the timer (see below), but the timer
interrupt never fires due to the added call to riscv_clock_event_stop(). You
need to move this line earlier in the function.

Here's the stack trace from the initial call to riscv_clock_next_event():
riscv_clock_next_event+0x12/0x3c
clockevents_program_event+0x9c/0x1c6
tick_setup_periodic+0x82/0x9e
tick_setup_device+0xa0/0xbe
tick_check_new_device+0x96/0xc6
clockevents_register_device+0xbe/0x128
clockevents_config_and_register+0x20/0x30
riscv_timer_starting_cpu+0xa2/0xec
cpuhp_invoke_callback+0x160/0x61e
cpuhp_issue_call+0x140/0x16e
__cpuhp_setup_state_cpuslocked+0x186/0x2b0
__cpuhp_setup_state+0x3a/0x60
riscv_timer_init_common+0xea/0x184
riscv_timer_init_dt+0xbe/0xc2
timer_probe+0x70/0xdc
time_init+0x74/0xa0
start_kernel+0x194/0x35e

Regards,
Samuel

> +
>  	enable_percpu_irq(riscv_clock_event_irq,
>  			  irq_get_trigger_type(riscv_clock_event_irq));
>  	return 0;